I. Field of the Invention
The present invention relates to solenoid operated pressure control valves employed in applications where the valve will accurately vary the pressure at a control port in accordance with variations in an electrical control signal, which may be derived from a computer, which varies the on-off time of energization of the solenoid.
II. Description of the Related Art
While there are many applications for such a valve, one application which has been of interest in recent years is that of the control of an automatic transmision for a motor vehicle by independently regulating the engagement pressure applied to each of the various clutches in the transmission. The torque transmitted by a given clutch may be varied by varying the pressure of engagement between the opposed clutch plates. Presently available electronic control units can rapidly and precisely generate the desired electrical output signals in response to sensed vehicle operating conditions. However, converting these electrical control signals into a precisely proportional fluid pressure which will accurately track variations in the electrical control signal has posed problems.
In such a system, a solenoid actuated valve is a logical choice as the interface between the electrical and hydraulic portions of the system. See, for example, U.S. Pat. No. 4,579,145 which describes a solenoid actuated valve for such an application. A system employing a valve of the type shown in that patent is described in some detail in SAE Technical Paper 840448.
As in U.S. Pat. No. 4,579,145 the solenoid actuated valve may be designed to regulate the pressure at a control port by cyclically connecting the control port alternately to a source of fluid under pressure and to a fluid sump, these alternate connections being made in accordance with the energization or deenergization of the solenoid coil. An electronic processor may be employed to regulate the time during each cycle the coil is energized ("on time"), the coil being deenergized for the remainder of the cycle ("off time"), this type of regualtion being commonly referred to as pulse width modulation. A typical operating pulse frequency might be 60 Hz. In steady state operation the pressure at the control port will be that percentage of the fluid source pressure which is equal to that precentage of time which the control port is connected to the fluid source, sump pressure being assumed to be zero.
In order to enable the control port pressure to be varied in a true linear relationship to variations in "on time" of the solenoid coil, the valve member which controls the fluid connection of the control port to pressure supply or sump must be capable of rapid shifting movement in close synchronism with the energization and deenergization of the coil. Further, the valve member should also be movable in response to a relatively small magnetic force in order to minimize the size and power requirements of the solenoid.
Conventionally, in solenoid actuated valves of this general type, a return spring is employed to bias the movable valve member to its solenoid deenergized position. The coil springs so employed pose handling problems in assembling the valve in that when they are supplied in bulk, the individual springs become entangled with each other and must be manually untangled. Good design practice dictates the usage of a minimum number of energy sources and the hydraulic and electrical energy sources should be sufficient.
The present invention is especially directed to a solenoid valve having these last characteristics.